1. Field of the Invention
The present invention relates generally to an electrophotographic image forming apparatus such as, for example, a copier or a printer, and more particularly to a developing bias power unit for applying a bias voltage to a developing sleeve which confronts a photosensitive member and supplies it with toner.
2. Description of the Prior Art
FIG. 1 schematically depicts a conventional developing bias power unit 104 for use in a reversaldevelopment type image forming apparatus 100. As shown in FIG. 2, the peripheral surface of a photosensitive member 101 is electrostatically charged at a predetermined voltage Vo, and the surface voltage of an area B on the photosensitive member 101 between respective image areas A and A' on the neighboring pages (which area B is hereinafter referred to as an inter-image area) is reduced to Vo' by illuminating the inter-image area B. The illumination of the inter-image area B is carried out by forcibly emitting a laser beam to regulate the intensity of the laser beam on the inter-image area B. Accordingly, the illumination of the inter-image area B does not aim at reducing the surface potential of the inter-image area B, but the surface potential of the inter-image area B is reduced to Vo' as a result of the forcible emission of the laser beam.
On the other hand, in a developing unit 102 for supplying the photosensitive member 101 with toner, a developing sleeve 103 confronting the photosensitive member 101 is connected with the developing bias power unit 104. When the developing sleeve 103 confronts one of the image areas A and A', a predetermined developing bias voltage V.sub.B is applied thereto by the developing bias power unit 104 to prevent the toner from adhering to a background area. When the developing sleeve 103 confronts the inter-image area B, no signal is output from the developing bias power unit 104 (developing bias voltage V.sub.B '=0V), thereby preventing toner adhesion to the inter-image area B.
However, the mere switching of the developing bias voltage V.sub.B ' over to 0V at the time the developing sleeve 103 confronts the inter-image area B results in only a voltage difference Vo' between the photosensitive member 101 and the developing sleeve 103. The voltage difference of this magnitude is not enough to prevent the toner adhesion to the inter-image area B.
Particularly, in a so-called contact-transfer type image forming apparatus wherein toner is transferred on a transfer material while the transfer material is sandwiched between the photosensitive member 101 and a transfer roller and is conveyed by the rotation thereof, if the toner adheres to the inter-image area B, the problem arises that the toner may be transferred to the transfer roller, or the toner adhering to the transfer roller may be transferred to and spoils the rear surface of the transfer material.
Prior to a printing operation, neither the developing sleeve 103 nor the photosensitive member 101 is electrostatically charged (0V), and there is no voltage difference therebetween. However, toner may adhere, more or less, to an uncharged area of the photosensitive member 101 that confronts the developing sleeve 103, because, prior to the printing operation, such an uncharged area is in contact with the toner at a nipping portion defined between the photosensitive member 101 and the developing sleeve 103. After the printing operation has been triggered and when the photosensitive member 101 starts rotating, an uncharged portion thereof lying between a charging region and a developing region is also in contact with the toner at the nipping portion. Because of this, even if there is no voltage difference between the photosensitive member 101 and the developing sleeve 103, the problem arises that the toner may adhere to such an uncharged portion to some extent.